1. Field Of The Invention
The present invention relates to a electroluminescent (sometimes referred to as "EL" hereinafter) device and, more particularly, to such a device having improved light output and a method of making the device.
2. Description Of Prior Art
Electroluminescent devices, whether for a light source, indicator lamp, or display applications have been known for many years. In such EL devices, electroluminescent materials are directly electrically excited to luminescence by the application of time varying electric fields to a volume containing electroluminescent phosphors contained between two electrodes, at least one of which is transparent.
Electroluminescent lamps are known principally in one or the other of two preferred embodiments. In the first, the so called "thick film" or "powder" EL, the electroluminescent material consists of a crystalline fine powder of activated electroluminescent phosphors such as zinc sulphide, cadmium sulphide, cadmium selenide, gallium phosphide, gallium arsenide and the like activated with copper, silver, manganese or others, dispersed in an insulating matrix which is disposed between a transparent conductor and a counter electrode.
The other principal type of electroluminescent lamp is the "thin film" EL, in which the electroluminescent material, again commonly activated EL phosphor compound, is vapor deposited in a thin film, sandwiched between two layers of similarly vapor deposited insulating material, with the entire assembly disposed between a transparent conductor and a counter electrode.
Regardless of the embodiment, EL lamps exhibit a strongly rising output of light with applied voltage and a nearly linear increase in output of light with increasing frequency. The output of light is very inefficient, however, being limited to a few lumens per watt in either embodiment. This low efficiency has prevented the widespread use of EL in any applications except for indicator or display devices.
It is widely accepted that this low efficiency stems from the fundamental physical phenomena occurring in these lamps, as follows: In the electroluminescence of such crystals as zinc sulfide, electrons liberated from deep traps or injected from contact electrodes or p-n junctions are accelerated by the electric field until they acquire sufficient kinetic energy to excite or ionize a luminescent center which subsequently radiates with the emission of light. A competing energy loss mechanism for the electrons is the loss of kinetic energy from elastic collisions between electrons and lattice atoms. Unlike the situation in a gas discharge, however, where electrons collide with isolated atoms unbound to others, the target atoms in a solid are bound by very strong elastic bonds to neighboring atoms. As a consequence, the target atom cannot move by itself upon impact, but must transmit some of this motion to adjacent atoms, which in turn transmit such motion to their neighbors, which in turn transmit such motion to their neighbors, etc, ad infinitum. Thus, the impact of an electron on a lattice atom, generates a sound wave travelling through the lattice. Such sound waves on the atomic scale are quantized, with energy in the wave packet =h.nu. where .nu. is frequency and h is Planck's constant 6.61.times.10.sup.-34 joule-sec. These quanta are called "phonons" by analogy with the photons of the optical spectra. The elastic collision of electrons with the lattice is therefore a collision with the phonon wave trains, and the interchange of electron energy with the phonon field is in units of the phonon energy. The problem is that in the usual electroluminescent solid of large spatial extent, the spectrum of lattice phonon waves is a very wide 1/f distribution (phonons per unit frequency interval vary inversely with frequency) in the manner of the solid curve in FIG. 1A. Thus there is a very large number of low energy phonon waves to which the electron may couple, losing small amounts of energy per collision, but with very frequent collisions of large cumulative energy loss. This large cumulative energy loss exerts a very large "drag" on the acceleration of electrons by the electric field. Moreover, the "cross-section" for collision with phonon waves varies approximately inversely with electron velocity, as in FIG. 1B. Accordingly, electrons at low energy, have a very large probability of losing energy to the plenitude of low energy phonon waves, which prevents them from gaining energy from the electric field to reach the energy range of reduced cross-section for collision, beyond which they experience reduced elastic collision loss and may be accelerated to energies capable of exciting or ionizing luminescent centers.
Consequently, as shown in FIG. 1C, the distribution of electron energies is strongly skewed to low energy electrons, even at high electric fields. Thus most of the electrons are unable to excite or ionize luminescent centers, but can only dissipate energy in phonon collisions that ultimately produce nothing more than heat. As a result, efficiency of electroluminescence is low, with efficacies of 1-10 lumens per watt, useful only for indicator or display devices.
Accordingly, a desirable object of the present invention is to provide an improved EL device and method of making the same.
Another desirable object of the present invention is to provide an EL device having higher efficiency and improved light output.
A still further desirable object of the present invention is to provide a thin or thick film EL layer structure which reduces energy losses of electrons in collision with the phonon waves of the lattice atoms, thereby increasing the fraction of the electron energy gained from the electric field dissipated in the useful excitation collisions with luminescent centers.
A still further desirable object of the invention is to accomplish the foregoing object by the substantial reduction or elimination of low frequency, long wavelength phonon waves with low phonon energy, to which low energy electrons may lose small amounts of energy per collision in very frequent collisions.
These and other desirable objects of the invention will in part appear hereinafter and will in part become apparent after consideration of the specification with reference to the accompanying drawings and the claims.